Steam generator

ABSTRACT

A vertical shell-and-tube heat exchanger of the type in which water is flown inside the tubes from a lower section while liquid sodium is flown outside the tubes from an upper section is provided. A liquid metal stable to both water and liquid sodium and greater in density than said liquid sodium is placed in a lower part outside the tubes of said heat exchanger thereby to improve heat efficiency and prevent sodium-water reaction caused by defect in the welded section of the lower tube plate, and water is heated and vaporized by liquid sodium.

United States Patent Naitou et al. July 1, 1975 [54] STEAM GENERATOR 3,628,507 12 1971 Saporiti 122 32 1 Masai-mi Nam; Himku Nam-o; 2125352 132333 521????fffjjjjiiiii: .111: 1533.3.

Shinichi Kashiwai; Chikara Satou; Motoaki Utamura, all of Hitachi, Japan Primary ExammerKenneth W. Sprague Attorney, Agent, or FirmCraig & Antonelli [73] Assignee: Hitachi, Ltd., Japan [22] Flled. Feb. 5, 1974 ABSTRACT [21] Appl. No.: 439,699

A vertical shell-and-tube heat exchanger of the type in D which water is flown inside the tubes from a lower sec- Appl'cauon Pnomy Dam tion while liquid sodium is flown outside the tubes Feb. I6, 1973 Japan 48 18265 from an upper section is provided. A liquid metal stable to both water and liquid sodium and greater in 5/135; /163 density than said liquid sodium is placed in a lower [Sl] Int. Cl. FZZb 1/06 part outside the tubes of aid heat exchanger thereby Field of Search 36, 153, to improve heat efficiency and prevent sodium-water 6 [22/32, 33, 34 reaction caused by defect in the welded section of the lower tube plate, and water is heated and vaporized by [56] References Cited li id di UNITED STATES PATENTS 3,395,676 8/1968 Sprague 122/32 15 4 Draw F'gures FEED WATER rm' gr u w 975 m892,205 SHEET 1 FIG. I

FEED WATER STEAM SODIUM f y 6 SODIUM H "4& I a; e L

A TL. 0

i l I 24% l9 5 l n I 9 wwgn JUL'l ms 3 8 92 2 O 5 SHEET 2 FIG. 2

FEED WATER SODIUM STEAM GENERATOR This invention relates to a steam generator adapted for use in a fast neutron reactor where liquid sodium is used as heating medium.

Heretofore, liquid sodium has been used as heating fluid in most of the steam generators for fast neutron reactors, Generally, an alkali metal such as sodium has a disposition to become reacted vehemently with water, or the object to be heated, to develop tremendous reaction heat while producing at the same time a great quantity of hydrogen. lt is therefore required to keep strict watch over leakage of water or sodium in the steam generator to avoid sodium-water reaction that becomes a source of troubles.

In most of the conventional steam generators, there is employed in the water supply header as well as in the outlet steam header section a construction where the tubes are welded to a thick-walled tube plate. In this case, particularly in the water supply header, if hightemperature sodium stays in contact with one side of the tube plate, large thermal stress is exerted to the tube plate portion as low-temperature water exists on the other side. Also, in certain types of construction of the welded section, corrosion may develop in the neighborhood of the tubes owing to active sodium. For these reasons, it is commonly practiced to let sodium, or the heating fluid, have a free liquid surface while disposing thereabove an inert gaseous layer and providing in this gaseous layer portion a water supply header and an outlet steam header which have the highest probability of leakage. These arrangements permit positive prevention of leakage of water from the tube plate section, and even if a slight amount of leakage should be caused, the reaction of such leak water with sodium takes place on the free surface of sodium below the gaseous layer, so that it is possible to inhibit the leakage propagation phenomenon. or the phenomenon that the reaction induced by initial leakage causes leakage from other portions. However, such construction is inevitably attended by formation of a downcast portion in each tube which is connected to the water supply header and to the steam header. Generally, presence of such downcast pipes is not only quite detrimental to performance of the steam generator but could also invite unstable water fluidity due to formation of voids by boiling and/or other factors if water temperature in such pipe is raised. Such fluid unstability leads to irregularity of the outlet steam temperature and fatigue of the tubes by heat cycle, and if such unstability of fluid takes place, operation of the steam generator must be immediately stopped. In order to avoid this, it has been generally practiced to provide a thermal shield with large thermal resistance in the inside of the downcast pipes or to incorporate other means to control rise of water temperature in the downcast pipes within in a certain limit.

Having the said arrangements and actions, the conventional steam generators had the following defects:

1. If feed water should be boiled while passing through the downcast pipes, supply of water is greatly impaired, so that it is necessary to provide a thermal shield plate or other like preventive means and to also keep strict watch over feed water temperature and flow rate. Therefore, such downcast pipe portion, although constituting a part of the heat exchanger, actually conduces little to the heat characteristic, and though it is of a construction capable of performing a slight degree of heat exchange, its heat efficiency is extremely low due to thermal resistance of the thermal shield plate. Also, sensible heat in the sodium portion at the bottom of the generator shell is not utilized effectively.

2. The water supply header and the steam header at the top of the steam generator are positioned closely adjacent to each other and their temperature difference is excessively wide (about 270C), so that the upper spherical head is always exposed to danger of thermal stress. Also, sodium coheres to the tube plate on the water feed side in long-time use, causing tremendous temperature difference (about 300C) corrosion by sodium, so that difficulties are raised over designing and maintenance of the tube plate on the water feed side.

3. Owing to the positional relations of the downcast pipes, tubes, thermal shield plate and two headers at the top of the apparatus, the general construction of the apparatus is extremely complicated and requires a number of steps and huge cost for manufacture thereof.

The object of the present invention is to provide an improved steam generator which is free of the described defects of the prior art and which is high in heat characteristic, simple and compact in structure, and easy of operation and maintenance.

For removing the defects of a steam generator having the downcast pipes such as aforesaid, it is best to device a structure which does not necessitate use of any downcast pipe. That is, if the water supply header is disposed at the bottom of the steam generator and the outlet steam header at the top thereof so that the water flow in the effective heat transfer section will be directed upwardly, it is possible to dispense with any downcast pipe and to remove all of the defects that originate from presence of such downcast pipes. However, in the structure where the water feed header is disposed at the bottom, if the header construction used is same as in the conventional systems, there is a greater danger of causing sodium-water reaction, and resultant troubles, due to possible water leakage from the tube plate section of the water supply header particularly when the the operation is continued for a long time. This is the very reason why the disposition of the water supply header at the botton of the steam generator has been tabooed. The present invention has succeeded in overcoming the problems that arise from such bottom-wise provision of the water supply header while allowing best use of the meritorious points of such arrangement.

The salient feature of the present invention resides in that liquid metal which is concordant with both sodium and water is disposed above the tube plate of the water supply header provided at the bottom of the steam generator, so as to prevent sodium from directly contacting with the tube plate section on the water supply side.

The advantageous effects obtained from the present invention may be summarized as follows:

1. An no downcast pipe is provided, there is no need of providing a thermal shield and means for controlling flow rate and temperature of feed water in the relevant section, thus allowing smooth heat exchange between sodium and feed water. Also, the generating surface area can be reduced (by about 15 percent) and sensible heat of sodium can be utilized to the maximum degree.

2. It is possible to avoid danger of producing large thermal stress would otherwise be produced in the spherical head at the top of the steam generator. Also,

it is uncessitated to device specific protective design and maintenance measures against cohesion and resultant corrosion of sodium to the tube plate particularly on the low-temperature water feed side.

3. Since no downcast pipe and thermal shielding plate are used and also arrangement of the tubes is simplified, the entire structure of the apparatus is accordingly compacted, allowing sizable reduction of the manufacturing cost.

FlG. l is a longitudinal sectional view of a conventional steam generator;

FIG. 2 is a partial sectional view of another conventional steam generator;

FIG. 3 is a longitudinal sectional view ofa steam generator according to the present invention; and

FIG. 4 is a temperature diagram of feed water and sodium in the conventional devices and the present invention.

The conventional devices are first described in detail with reference to the relevant drawings.

Referring to HO. 1, there is shown, in longitudinal section, a conventional steam generator comprising the shell l, a tube plate 3 on the water supply side, a water supply header 5, a gas side flange 6, an outlet steam header 7, an upper tube plate 8, a tube supporting structure 9, downcast pipes 10, tubes 11, a sodium inlet nozzle 12, a sodium outlet nozzle 13, a thermal shielding plate 19, a feed water layer 21, an inlet nozzle of feed water 21a, a steam layer 22, a steam discharge nozzle 220, a sodium layer 24, an inert gas layer 25, an inert gas feed nozzle 25a, and an inert gas dicharge nozzle 25b. Letter A indicates the position of feed water in the downcast pipes 10 which stay in the interface between the inert gas layer 25 and the sodium layer 24, a the sodium of the same interface thereof, B the position of feed water at the bottommost end of the downcast pipes 10, b the position of sodium 24 at the same level as B, C the position of steam in the tubes 11 at said sodium interface, the interface thereof, and d the sodium in the sodium outlet nozzle 13.

A plurality of downcast pipes 10 extend downwardly along the outside of the thennal shielding plate 19 from the tube plate 3 of the water supply header and are turned back at the bottom of the shell 1 to extend now upwardly to serve as tubes while helically surrounding the thermal shielding plate 19 and the tube supporting structure 9 with the aid of a mounting seat (not shown) disposed at an outer peripheral part of said supporting structure 9 which is closed at its top and bottom, and said tubes are finally converged in the outlet steam header 7. ln order to avoid corrosion by sodium, said plurality of downcast pipes and tubes 11 are formed from seamless low carbon steel pipes (for use in high temperatures) or seamless stainless steel pipes (for use in low temperatures) arranged with same length, and the tube supporting structure 9 and thermal shield plate 19 are retained in position along with said downcast pipes l0, tubes ll and sodium inlet nozzles 12 by the tube plates 3, 8 and gas side flange 6.

In a steam generator adapted for instance in a fast breeder with thermal output of 250 MW, feed water is supplied to the header 5 under pressure of around 175 kg/cm g at 240C, and such feed water flows down in the downcast pipes 10 until it reaches the bottommost end B. Upon reaching this end point B, said feed water now rises up in the tubes 11, and in the course of this ascent, said water is heated by sodium and then collected in the steam header 7 in the form of superheated vapor with pressure of around kg/cm' g and temperature of about 513C. In the meanwhile, sodium of approximately 540C is supplied from the inlet nozzle l2 and descends around the tubes 11 within the thermal shield 19 while heating water in said tubes. Through this heat exchange, said sodium is cooled to about 300C and then discharged out from the sodium outlet nozzle 13. ln the space at the top of the shell 1 is provided an inert gas layer 25, and the sodium liquid levels a and c are always maintained constant.

In the primitive step of development of steam generators, there was also proposed a construction where the tubes themselves are arranged double. FIG. 2 shows, in section, the water supply header and its vicinity in a steam generator using such double-pipe arrangement. In the figure, reference numeral 1 indicates the shell of the steam generator, 3 a tube plate on the water side, 5 a water supply header, ll tubes, 12 a sodium inlet nozzle, 13 a sodium outlet nozzle, 14 a mercury inlet nozzle, 21 a feed water layer, 21a a feed water inlet nozzle, 23 a mercury layer, and 31 outer pipes.

Mercury is supplied from the inlet nozzle 14 and charged into the outer pipes around the tubes 11 to form a mercury layer 23, while high-temperature sodium is supplied from the inlet nozzle 12 and discharged out, after cooled, from the outlet nozzle 13. In the meantime, feed water enters the header 5 through the nozzle 21a and passes through the tubes 11, and during passage through said tubes, it is heated by a high-temperature sodium and is withdrawn in the form of high-temperature steam from a steam header (not shown) at the other end.

Application of such double-pipe arrangement along the entire length of the tubes is intended to reduce thermal stress in the tube walls with the aid of an intermediate mercury layer. That is, in a single tube arrangement, large thermal stress is exerted to the tube wall as low-temperature feed water and high-temperature sodium exist on both sides of the tube wall, and such thermal stress, when accumulated, may burst the tube wall. The double-pipe arrangement can prevent such trouble; However, this arrangement has a serious drawback that thennal resistance is increased by the presence of outer pipes and intermediate mercury layers, thus causing excessive reduction of the heat efficiency.

Now, the present invention is described in detail by way of a preferred embodiment thereof with reference to the relevant drawings.

Referring to FIG. 3, there is shown, in longitudinal section, a steam generator embodying the present invention. In the figure, reference numeral 1 designates the shell of the steam generator, 2 a heating medium reservoir tank, 3 a lower tube plate, 4 a flange, 5 a water supply header, 6 a gas side flange, 7 an outlet steam header, 8 an upper tube plate, 9 a tube supporting structure, 11 tubes, 12 a sodium inlet nozzle, 13 a sodium outlet noule, 14 a heating medium inlet nozzle, 15 a heating medium outlet nozzle, 21 a feed water layer, 21a an inlet nozzle of feed water, 22 a steam layer, 220 a steam outlet nozzle, 23 a heating medium layer, 24 a sodium layer, 25 an inert gas layer, 25a an inert gas feed nozzle, and 25b an inert gas discharge nozzle.

The tube supporting structure 9 per se and its operational relation with the tube 11 are same as those in conventional apparatus shown in FIG. 1. The salient structural differences of the present invention from the conventional apparatus shown in FIG. 1 are that the water supply header 5 is provided at the bottom of the generator body and the steam header 7 at the top thereof, with said both headers being connected respectively through a flange to a plurality of tubes arranged with equal length coiling around the tube sup porting structure 9, and that a layer of a heating medium 23 such as mercury is provided between the water feed side flange and feed sodium. The heating medium used here is a substance which is not dissolved, diffused or emulfied in (that is, physically inert to) both water and sodium and is also not chemically reacted with (that is, chemically inert to) water and sodium and which has a melting point of lower than 300C and is greater in density than sodium. Although mercury is used for such heating medium in the embodiment discussed hcre, the heating medium usable in the present invention is of course not limited to mercury; it is possible to use molten metal such as bismuth or indium or their alloys, or stabilized alloys thereof with sodium. The tube supporting structure 9 is adapted to securely fix the tubes arranged helically therearound and is closed at its both upper and lower ends.

Firstly, mercury is supplied into the mercury reservoir 2 in an amount of about 70 percent so that it will not be affected by the flow sodium, then sodium is charged into the steam generator and an inert gas is further supplied thereabove to thereby fill the steam generator. In the meanwhile, high-temperature sodium is supplied from the inlet nozzle 12 and flown down passing between the tubes 11 and discharged out from the outlet nozzle 13 at the bottom of the shell 1. As sodium flows down as a one-way current and its central portion is also occupied by the tube supporting structure and the tubes, there is no reason for a chance of disturbing the flow of sodium in the vicinity of the mercury reservoir 2. On the other hand, feed water supplied to the feed water header 5 flows upward through the mercury layer 23 and the sodium layer 24 into the tubes 11 where said water is further heated while swirling and boiled to produce steam which is once collected in the steam header 7 and then released out therefrom.

In the figure, d refers to the bottom surface of the heating medium layer, d the interface of the heating medium and sodium, and C the interface of sodium an inert gas. D, D and C indicate the positions of feed water corresponding to said respective interfaces.

FIG. 4 shows a temperature diagram of the present invention in comparison with the prior art apparatus (FIG. 1).

In the prior art apparatus, temperature transition of feed water traces the course A B C and that of sodium follows the course a b c. The segment A B indicates temperature transition in the downcast pipes, with the point B signifying the bottommost end of the downcast pipes. Corresponding sodium temperature is also low as indicated by b.

Although feed water in the section A B serves as heat exchanger, it is rather required to provide a detection-control system for restraining rise of temperature of feed water under a certain level. Further, temperature difference between sodium and feed water in this section is reversed and also their flows are parallel, so that this section is not in a situation which permits performance of normal heat exchange. The segment B C represents temperature transition of feed water in the tubes and corresponding temperature transition of sodium is indicated by segment b c. In this section, temperature difference between them is maintained substantially constant and also their flows are counter current, so that ideal heat exchange can be effected in this section.

In comparison with this, in the steam generator according to the present invention, temperature of feed water rises up following the course D D B C while sodium flows counter to feed water as indicated by line c* b*d a", and their temperature difference can be maintained substantially constant with ease.

Also, in the conventional apparatus, sensible heat of sodium was utilized merely in the temperature section c b, but in the present invention, such sensible heat can be more effectively utilized in the section b d (temperature of the heating medium layer is also utilized in the section (1 (1').

Further, in the conventional apparatus, the heating surface area expands wide from I to V (in FIG. 4) and a part thereof is actually little utilized, whereas in the present invention such area is limited to the section I-IV, allowing about 15 percent reduction.

While the present invention has been described by way of an embodiment where the headers are provided at both top and bottom and the tubes are coiled helically around a support structure, with the entire unit being vertical, the present invention is not limited to such particular embodiment; it sufficis if the entire unit is arranged vertically such that heat exchange between sodium and water is accomplished in a counter current condition and sodium and water are sealed at the bottom through a liquid metal which can coexist stably with said sodium and water.

Also, in the foregoing description of the present invention adapted as a steam generator for a fast breeder, liquid sodium was nominated as the heating medium while water as the object to be heated, but such heating medium and object to be heated are not limited to sodium and water, respectively. It is possible to use any other suitable combination of liquids which are well reactive to each other.

What is claimed is l. A steam generator of a vertical multitubular (bushing) type having at least one tube plate at the lower end of the tubes and using two fluids flowing inside and outside said tubes respectively and reacted by contact with each other.

characterized by a reservoir located outside said tubes and on-said tube plate at the lower end of said tubes, and an intermediate heating medium disposed in said reservoir, said medium being physicochemically inert to said both fluids, and is greater in density than said both fluids, and has melting point of not higher than 300C.

2. The steam generator according to claim 1, wherein said fluid flowing inside the tubes is water and said fluid flowing outside said tubes is sodium, and said intennediate heating medium is mercury.

3. The steam generator according to claim 2, wherein said heating medium is bismuth.

4. The steam generator according to claim 2, wherein said heating medium is indium.

5. The steam generator according to claim 2, wherein said heating medium is an alloy composed of a combination of two or more of sodium, mercury, bismuth, lead and indium.

6. A steam generator of a vertical multitubular type, comprising a shell, a heating fluid inlet nozzle and inert gas feed and discharge nozzles provided at the top of said shell, an outlet nozzle for said heating fluid provided at the bottom of said shell, a vapor header and a feed liquid header provided at the top and bottom, respectively, of said shell, upper and lower tube plates mounted to said respective headers, a plurality of helically arranged tubes connected through said respective tube plates to said respective headers, a support structure including a cylindrical body for supporting said plurality of helical tubes, and a reservoir of an intermediate heating medium which is physico-chemically inert to said both feed liquid and heating fluid and which is greater in density than said heating fluid and has melting point of lower than 300C, said reservoir being disposed on said lower tube plate so as to enclose fixed portions of said tubes and said tube plate in a bundle.

7. The steam generator according to claim 6, wherein said heating fluid is sodium, said feed liquid is water, and said intermediate heating medium is mercury.

8. The steam generator according to claim 7, wherein said intermediate heating medium is bismuth.

9. The steam generator according to claim 7, wherein said heating medium is indium.

10. The steam generator according to claim 7, wherein said heating medium is an alloy made of a combination of two or more of sodium, mercury, bismuth, lead and indium.

11. A steam generator ofa vertical multitubular type,

comprising a shell, a heating fluid inlet nozzle and inert gas feed and discharge nozzles provided at the top of said shell, an outlet nozzle for said heating fluid provided at the bottom of said shell, a vapor header and a feed liquid header provided at the top and bottom, respectively, of said shell, upper and lower tube plates mounted to said respective headers, a plurality of helically arranged tubes connected through said respective tube plates to said respective headers, a support structure including a cylindrical body for supporting said plurality of helical pipes, and a cylindrical reservoir of an intermediate heating medium which is physicochemically inert to said both feed liquid and heating fluid and which is greater in density than said both feed liquid and heating fluid and has melting point of lower than 300C, said cylindrical reservoir disposed on said lower tube plate so as to enclose fixed portions of said tubes and said tube plate in a bundle.

12. The steam generator according to claim 11, wherein said heating fluid is sodium, said feed liquid is water, and said intermediate heating medium is mercury.

13. The steam generator according to claim 12, wherein said intermediate heating medium is bismuth.

14. The steam generator according to claim 12, wherein said heating medium is indium.

15. The steam generator according to claim 12, wherein said heating medium is an alloy composed of a combination of two or more of sodium, mercury, bismuth, lead and indium. 

1. A steam generator of a vertical multitubular (bushing) type having at least one tube plate at the lower end of the tubes and using two fluids flowing inside and outside said tubes respectively and reacted by contact with each other. characterized by a reservoir located outside said tubes and on said tube plate at the lower end of said tubes, and an intermediate heating medium disposed in said reservoir, said medium being physico-chemically inert to said both fluids, and is greater in density than said both fluids, and has melting point of not higher than 300*C.
 2. The steam generator according to claim 1, wherein said fluid flowing inside the tubes is water and said fluid flowing outside said tubes is sodium, and said intermediate heating medium is mercury.
 3. The steam generator according to claim 2, wherein said heating medium is bismuth.
 4. The steam generator according to claim 2, wherein said heating medium is indium.
 5. The steam generator according to claim 2, wherein said heating medium is an alloy composed of a combination of two or more of sodium, mercury, bismuth, lead and indium.
 6. A steam generator of a vertical multitubular type, comprising a shell, a heating fluid inlet nozzle and inert gas feed and discharge nozzles provided at the top of said shell, an outlet nozzle for said heating fluid provided at the bottom of said shell, a vapor header and a feed liquid header provided at the top and bottom, respectively, of said shell, upper and lower tube plates mounted to said respective headers, a plurality of helically arranged tubes connected through said respective tube plates to said respective headers, a support structure including a cylindrical body for supporting said plurality of helical tubes, and a reservoir of an intermediate heating medium which is physico-chemically inert to said both feed liquid and heating fluid and which is greater in density than said heating fluid and has melting point of lower than 300*C, said reservoir being disposed on said lower tube plate so as to enclose fixed portions of said tubes and said tube plate in a bundle.
 7. The steam generator according to claim 6, wherein said heating fluid is sodium, said feed liquid is water, and said intermediate heating medium is mercury.
 8. The steam generator according to claim 7, wherein said intermediate heating medium is bismuth.
 9. The steam generator according to claim 7, wherein said heating medium is indium.
 10. The steam generator according to claim 7, wherein said heating medium is an alloy made of a combination of two or more of sodium, mercury, bismuth, lead and indium.
 11. A steam generator of a vertical multitubular type, comprising a shell, a heating fluid inlet nozzle and iNert gas feed and discharge nozzles provided at the top of said shell, an outlet nozzle for said heating fluid provided at the bottom of said shell, a vapor header and a feed liquid header provided at the top and bottom, respectively, of said shell, upper and lower tube plates mounted to said respective headers, a plurality of helically arranged tubes connected through said respective tube plates to said respective headers, a support structure including a cylindrical body for supporting said plurality of helical pipes, and a cylindrical reservoir of an intermediate heating medium which is physico-chemically inert to said both feed liquid and heating fluid and which is greater in density than said both feed liquid and heating fluid and has melting point of lower than 300*C, said cylindrical reservoir disposed on said lower tube plate so as to enclose fixed portions of said tubes and said tube plate in a bundle.
 12. The steam generator according to claim 11, wherein said heating fluid is sodium, said feed liquid is water, and said intermediate heating medium is mercury.
 13. The steam generator according to claim 12, wherein said intermediate heating medium is bismuth.
 14. The steam generator according to claim 12, wherein said heating medium is indium.
 15. The steam generator according to claim 12, wherein said heating medium is an alloy composed of a combination of two or more of sodium, mercury, bismuth, lead and indium. 